Flat straps are used for many applications that typically require one or both ends of the flat strap to be secured in some manner. Tying knots in flat straps are particularly troublesome because they can be incredibly difficult to untie once tension has been applied to the knot. Often the remedy is to cut the knot from the flat strap. Given that the flat strap typically is much wider than it is thick, attempting to use flat straps in a manner similar to ropes can be impractical. Flat straps are very flexible, resistant to stretch, typically stronger than rope and, compared to rope, usually much friendlier in how they interact with objects bound by them.
In most applications that utilize flat straps to secure or bind objects, the incorporation of an ancillary device to manage the tightening and/or binding of the flat strap is required. Most devices designed for flat straps take advantage of the greater strength of flat straps and, therefore, provide means to increase the binding tension applied to the load beyond what one could manually achieve. Also, these devices attempt to provide repeatability and convenience in terms of set-up and use. Two particular types of devices are most prevalent. They are commonly known as ratchet devices and cam-lock devices.
Ratchet devices find use in high load applications where supporting or inducing a large amount of tension in the flat strap is required—usually far above what a normal person would be capable of generating. To achieve this level of tension, some form of mechanical advantage or amplification of force is required. Ratchet devices do this by utilizing a lever implemented in a manner where the flat strap is ‘levered’ into the device and wound around a drum. To prevent the device from losing the induced tension in the flat strap, a ratchet scheme is used. Small increments of rotation of the tensioning lever are preserved by a directional locking scheme or ratchet action. Typically, several components are required in the construction of a ratchet device—toothed ratchet plates, springs, lockout and release brackets, a center drum, a lever/handle, etc. The primary advantage of a ratchet device is the high level of tension that can be generated in the flat strap. However, ratchets often are complicated devices both in terms of construction and in their use. They are limited in the amount of flat strap they can accommodate on the drum, frequently requiring repeated resets of the device or a significant amount of pre-tensioning or manual cinching of the load prior to use. Also, it is not uncommon for the flat strap to misalign or foul as it is drawn into the device, which can easily render the device unusable.
Cam-lock devices are used in lower load applications and in applications where ease of use, cost, and simplicity are important. A cam-lock device typically is comprised of a small frame or chassis, a locking mechanism commonly incorporating a torsion spring, and a shaft or swage pin to attach the locking component and spring to the chassis. Integrated into the chassis is some kind of static drum or wrap bar over which the flat strap is drawn into the device. Similar to ratchet devices, cam-lock devices are directional. Cam-lock devices allow the flat strap to be drawn into and through the device but prevent the flat strap from reversing back out of the device. The ‘cam’ designation typically comes from the manner in which the component(s) are used in preventing reversal of the flat strap. As more tension is applied to the flat strap, there is a stronger impulse for the strap to be drawn back through the device.
To accommodate the increase in tension, the cam force or locking action must increase proportionally. The particular shape of the cam or locking component(s) are such that, as tension in the flat strap increases, the cam is drawn tighter to the wrap bar, or the gap through which the flat strap is held in place is made smaller. Additionally, the release of the locked ‘cam’ usually requires only overcoming forces normal (perpendicular) to the tension in the flat strap rather than overcoming the tension in the flat strap directly. Quite common in the design of a cam-lock device is the use of a torsion spring to press the locking component into the flat strap. The locking component usually incorporates some form of teeth, points, or grip geometries to help initiate and sustain the ‘cam-action’ as the flat strap attempts to back out of the device. The torsion spring does not contribute to the locking force of the device per se; rather, it helps to insure that the locking component initiates engagement of the flat strap.
The primary advantage of a cam-lock device is ease of use. The user presses on one end of the locking component (usually configured as a lever to generate mechanical advantage) to lift the grip portion of the component away from the drum or wrap bar and feed the flat strap into the device. Once the flat strap is fed into the device, the spring loaded lever is released, allowing it to press against the flat strap. To tighten or secure the flat strap, the user simply draws the strap through the device. To release the device, the user presses the lever, lifting it away from the flat strap.
There are several limitations or disadvantages in using cam-lock devices. At high loads, releasing the locking device can result in a violent movement of the strap/device. Most users familiar with cam-lock devices have learned to be quite attentive when they release the device.
Often the limitations of a particular cam-lock device stem from how the device is implemented. It is not uncommon for manufacturers to route the flat strap through a sharp transition (around an edge of the device) before routing the flat strap through the locking portion of the device. Doing this severely weakens the flat strap and is a key limiter in the load which the device can be rated to support—and usually is significantly lower than the rated capacity of the flat strap. This is particularly common in inexpensive devices that utilize metal stampings for the device chassis.
Another common limitation is that the locking action often is confined to a small area of the flat strap. This is required in order to generate enough locking force to thoroughly engage the cam action to prevent the flat strap from slipping through the device, or it is the result of a design that has no capability to distribute the locking action in any other way. For example, a ‘single-line’ contact formed between two cylinders (the wrap bar portion of the chassis and the locking lever) is the most prevalent design reason for the limitation. The down side to this, besides the flat strap slipping, is similar to the issue described above—the highly localized stress applied to the flat strap can severely limit the load bearing capacity of the flat strap.
Another malady of cam-lock devices concerns the flat strap itself. Usually, no provisions are made to address how the flat strap exits the device—leaving the locking component (release lever) and/or the flat strap exposed and vulnerable to damage or inadvertent release. The remedy to this leaves the user to tie the free end of the flat strap around the device or to tie the free end around the flat strap under load—arguably defeating the purpose of using a cam-lock device in the first place.
Also, while providing some leverage in assisting the tensioning of the flat strap, cam-lock devices typically are limited to a maximum 2-to-1 mechanical advantage—that being the result of drawing the flat strap around the fixed drum or primary wrap bar.
Obviously, both types of flat strap devices, the ratchet and the cam-lock, have found wide use in the world. However, people often have to make compromises in the use of either device.